de461fa47c
Use subject instead of invalid_subject because the validation assertions test the subject double. This also adds an additional check when validating the `size` attribute because it is only required for disks of type `:disk`.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.